Magnifying parts of diagnostic images is important for medical image interpretation, as it allows a better view of anatomical structures relevant for diagnostic purposes. In the case of conventional X-ray film, this was achieved by means of a magnifying glass which was held and moved around in front of the film on a light-box. In the digital age, many image viewing applications offer pan and zoom functionality to enable a user to select a portion of an image for viewing at a selected magnification or zoom level. Medical imaging applications may offer pan and zoom functionality to enable a user to analyze the images and any pathologies visible in the images in a greater detail. The zoom functionality is also used for other kinds of images, such as geographic maps in navigation systems. Typical imaging applications use zoom and pan functionality in order to allow a user to expose places of interest in the image and to show details of those places. Zoom (enlargement) and pan (translation) are considered basic operations to imaging applications, and therefore they are frequently used during an imaging interpretation session.
In existing image viewing applications, the zoom operations are typically oriented to the center of the viewport: during zoom-in and zoom-out operations, the center of the viewport is kept stationary. In other words, the image point which is displayed at the center of the viewport remains at the center, while the remaining image points diverge away from the center or converge towards the center. Alternatively, a point in the image is selected using a mouse pointer, and this point is kept fixed during the zoom operation. The remaining points diverge away from this point or converge towards this point. This means that an indicated pixel remains fixed, while the other image pixels move away from (for zoom-in) or towards (for zoom out) that pixel.
However, during a zoom-out operation, the image may move partially or completely out of view. This way, the display area is not fully used. Moreover, additional panning may be necessary to view a larger portion of the image. Some existing image viewing applications attempt to prevent that the viewport is not fully used; however, these image viewing applications change the direction in which the image pixels are moved during the zoom operation as soon as an image boundary appears in the viewport. This may be disturbing for a user.